This invention relates generally to the ozonization of water for disinfection and algae control.
Ozonization of water for disinfection and algae control is well-established in Europe and Japan. The use of this method of water treatment is expected to expand world-wide, and particularly in the United States, because ozone (O.sub.3) is considered to be an environmentally-friendly biocide that produces no hazardous by-products when used alone. The impending availability of economic and efficient ozone generating systems will increase the use of ozone in water treatment, generally.
More particularly, ozone is presently in use in the treatment of recreational waters, such as swimming pool and spa waters, in the treatment of recirculating cooling water, and in the treatment of wastewater. In these applications, however, ozone suffers some widely recognized limitations: it is unstable in water, and it is readily volatilized from water. Consequently, it is difficult for an ozone-based water treatment system to maintain a residual level of ozone, which is often necessary for effective disinfection. The traditional strategy that has been used to overcome these ozone limitations is to post-treat water that has been ozonized with a conventional chlorine method. Alternatively, to ensure a residual level of ozone in the bulk of the water system, a high capacity ozone generator may be employed.
It has been recognized that the presence of the bromide ion (Br.sup.-) in water that has been ozonized results in the formation of hypobromous acid (HOBr), which is a secondary oxidizing biocide. Since hypobromous acid is more persistent in water than ozone, the hypobromous acid confers residual disinfection. Therefore, when the bromide ion is present during ozonation of water, the traditional post-treatment chlorine practice may be eliminated, along with the hazards associated with the storage and handling of chlorine or chlorinated chemicals.
Several papers have been published describing the interaction of ozone with the bromide ion, and many ozonization conditions have been considered, ranging from the ozonization of seawater (65 ppm Br.sup.- ion), to the ozonization of river water (10 ppb Br.sup.- ion). The kinetics and mechanism of the reactions of the bromide ion in the ozonization of water, and the potential for trihalomethane (THM) formation, have also been addressed.
The generally accepted mechanism for the interaction of ozone with the bromide ion has been provided by W. R. Haag and J. Hoigne, "Ozonization of Bromide-Containing Waters: Kinetics of Formation of Hypobromous Acid and Bromate," Environ. Sci. Technol., 17(5), 261, 1983; and W. R. Haag and J. Hoigne, "Kinetics and Products of the Reactions of Ozone with Various Forms of Chlorine and Bromine in Water," Ozone Sci. & Eng., 6, 103, 1984. Haag and Hoigne established that the reaction of ozone with the chlorine ion (Cl.sup.-) is unproductively slow, but interesting chemistry was revealed with the bromide ion, which is summarized in FIG. 1.
Ozone oxidizes the bromide ion at a moderate rate, initially to hypobromous acid (HOBr), which only reacts further in its hypobromite ion (OBr.sup.-) form with ozone. Two pathways were identified by Haag and Hoigne for further reaction of ozone with the hypobromite ion: EQU O.sub.3 +OBr.sup.- .fwdarw.O.sub.2 +Br.sup.- ( 1) EQU 2O.sub.3 +OBr.sup.- .fwdarw.2O.sub.2 +BrO.sub.3.sup.- ( 2)
The first reaction represents a catalytic decomposition of ozone with regeneration of the bromide ion, and the second reaction results in the formation of a highly undesirable bromate ion (BrO.sub.3 -.sup.-) by-product that removes a bromide ion from the cycle. Consequently, on prolonged ozonization, all the bromide ions materialize as bromate. One of the biggest problems facing the ozone-treatment of drinking water is that ozone reacts with naturally-occurring bromide ion (Br.sup.-) to produce the highly undesirable bromate ion (BrO.sub.3.sup.-).
Both reactions (1) and (2) are a waste of ozone. Nevertheless, it is apparent that the addition of bromide ions to water that has been ozonized can result in the formation of hypobromous acid as a persistent oxidizing biocide.
In practice, the moderate rate of reaction between ozone and the bromide ion means primary disinfection is accomplished by ozone, and secondary, or residual, disinfection by hypobromous acid, generated when the water acquires low microbial populations. This property of "internal generation" of a residual disinfectant renders post-ozonization chlorine treatment obsolete, and permits the use of smaller, and thus cheaper, less energy intensive, ozone generators for a particular ozonation application.
The ozone/bromide ion system has been in use in Europe for swimming pool water treatment. There is also a report of a German recirculating cooling water tower that has become more efficient when bromide ion is deliberately added to the ozonated cooling water. In these known applications, it would appear that the ozone-loss reactions (1) and (2) are tolerated, and that bromate production is not an issue.